In order to ensure good yield in a semiconductor fabrication process and the reliability of devices, it is indispensable to use a total clean-up technique throughout the fabrication process. This necessitates clean-up in fabrication steps and cleaning the surface of a wafer. One of control points necessary for cleaning the surface of a silicon wafer is the amount of metallic impurities. The metallic impurities on the silicon wafer are closely related to device characteristics. Metallic impurities can cause a junction leakage current to increase and the breakdown voltage of a gate oxide to degrade, and can have a significant effect on the reliability of devices.
In terms of the reliability of semiconductor devices and improvement in yield of semiconductor devices, it is extremely important that the amount of metallic impurities on a silicon wafer be analyzed with precision, and that the relationship between the amount of the metallic impurities and devices characteristics is determined.
As a method to determine the amount of metallic impurities on a semiconductor (Si) wafer with an oxide film (SiO.sub.2), a method wherein impurities on the silicon wafer are decomposed by HF vapor or an HF solution and are collected, and then the amount of the respective impurities in the collected solution is determined with flameless atomic absorption spectrometry or inductively coupled plasma mass spectrometry has been proposed, for instance.
As a method for recovering impurities on a silicon wafer, there has been proposed vapor phase cracking wherein a silicon wafer with an oxide film and an HF solution are sealed in a sealed vessel to make the oxide film react with the HF vapor for a certain period of time so as to decompose impurities, and the reaction solution is recovered (JP-A-2192750 and JP-A-5283381).
If the silicon wafer is arranged in a vertical position in the vapor phase cracking, the reaction solution containing the impurities drips. The dripping solution can be automatically collected in a receiver for a reaction solution under the silicon wafer to recover the impurities on the entire surface of the silicon wafer ("Analytical Chemistry" Vol.37 page 215 (1998)).
If a silicon wafer has a naturally formed oxide film, a step for forming a thermal oxide film before decomposition by HF vapor is provided to make the formation of a dripping reaction solution easy for collecting the solution ("Extended Abstract of the 16th Conference on Solid State Devices and Materials" A. Shimazaki, H. Hiratsuka, Y. Matsushita and A. Yoshii p.281 1994). Or, if there is no step of forming the thermal oxide film, a small amount of ultrapure water or HF solution is dripped on a silicon wafer after decomposition by HF vapor, and the dripped solution is made to scan the entire surface of the silicon wafer to recover the impurities ("Analytical Chemistry" Vol.38 page 177, 1989).
There has been proposed liquid phase cracking wherein a small amount of HF solution is dripped on a silicon wafer with an oxide film thereon, the solution is left for a certain period of time to react with the oxide film on the entire surface of the silicon wafer so as to decompose impurities, and the reaction liquid which gathers at a single location on the silicon wafer is recovered by a micropipette (JP-A-2272359). If a silicon wafer has a naturally formed oxide film, an HF solution is made to scan the entire surface of the silicon wafer to decompose impurities, recovering the impurities (JP-A-3239343).
The principles and systems for the flameless atomic absorption spectrometry and inductively coupled plasma mass spectrometry are described in detail in articles such as "Furnace Atomic Absorption Spectrometry-determine an infinitesimal amount" pages 1-56, issued by Gakkai Shuppan Center and "Plasma Ion Source Mass Spectrometry" pages 13-43, issued by Gakkai Shuppan Center.
If impurities on a silicon wafer with an oxide film thereon are recovered, a dripped reaction solution can be automatically collected to recover the impurities from the entire surface of the silicon wafer in either an HF vapor recovery or an HF solution recovery.
However, if impurities on a silicon wafer with a naturally formed oxide film thereon are recovered, no dripped reaction solution is formed with HF vapor. Even if an HF solution is dripped, the dripped solution scatters, making recovery of the impurities difficult.
In order to cope with this problem, it is necessary to form a thermal oxide film to increase the thickness of the oxide film so as to make recovery of a dripped reaction solution easier. However, there is a possibility that additional contaminants may get mixed with the oxide film from an ambient gas or impurities are vaporized in a step for oxidation. If there is provided no step for thermal oxidation, it is necessary to make an HF solution or ultrapure water scan the entire surface of a silicon wafer to recover impurities. In this case, it is necessary to scan the surface of the silicon wafer from edge to edge, and recovering the impurities without making the dripped solution separate from the silicon wafer requires considerable practice. The time and the number of scans by the dripped solution vary from one portion to another portion on the wafer, making uniform recovery extremely difficult.
The impurities which should be included in the amount of impurities on a silicon wafer are impurities which have been originally included in a surface of the silicon wafer and contaminants which are typically received in the surface of the silicon wafer in a semiconductor fabrication process. Contaminants which are locally received in a peripheral portion of a silicon wafer by contact of the wafer with a transportation carrier or a pair of tweezers should not be included in the impurity analysis. However, there is a possibility that the analysis is carried out in such a way so as to include local impurities because the recovery is carried out in such a way to include the impurities on the surface of a silicon wafer at its peripheral portion where the wafer contacts with a transportation carrier or a pair of tweezers. Under the circumstances, it is presumed that a positive error is included in the analysis because of failure to obtain a determinative value as to the amount of impurities. It is difficult to carry out a quantitative analysis with sufficient precision.